1. Field of the Invention
The present invention relates to an image pickup apparatus used in a digital camera or the like, and is structured to have plural image pickup areas used to pick up an image of a object by dividing the image into plural areas and plural output portions provided to correspond to the respective image pickup areas. Particularly, the present invention relates to a method which automatically discriminates output levels among plural outputs and corrects the output levels so as to eliminate unbalance in the plural outputs.
2. Related Background Art
Conventionally, a digital still camera having such a structure as shown in FIG. 1 has been known. In the case of the structure shown in FIG. 1, a system control CPU 100 detects a state change of a camera operation switch 101 (composed of a main switch and a release switch of a camera) caused by a photographer himself, and starts power supplying to other circuit blocks.
An image of an object within a photographing picture range is formed on an image pickup element 104 through main photographing optical systems 102 and 103, and an electrical signal from the image pickup element 104 is converted into a predetermined digital signal for each pixel in due order by an A/D conversion circuit 106 through a CDS/AGC circuit 105.
On the basis of a signal from a timing generator 108 which determines overall driving timing, the image pickup element 104 is driven by an output of a driver circuit 107 which horizontally and vertically drives each pixel, and thus the image pickup element 104 outputs an image signal.
Similarly, the CDS/AGC circuit 105, which performs analog processing to the output from the image pickup element 104, to convert its level into a predetermined signal level, and the A/D conversion circuit 106 operate on the basis of the timing from the timing generator 108.
An output from the A/D conversion circuit 106 is input to a memory controller 115 through a selector 109 which performs signal selection based on a signal from the system control CPU 100, and all signal outputs from the controller 115 are transferred to a frame memory 116. Therefore, since pixel data for each photographing frame is first stored in the frame memory 116 in this case, running (or successive) photographing or the like is performed as a writing operation to the frame memory 116.
After the photographing ends, the contents of the frame memory 116 which stores the photographed data are transferred to a camera DSP 110 through the selector 109 under the control of the memory controller 115. The camera DSP 110 generates R, G and B color signals on the basis of the pixel data of the photographed data stored in the frame memory 116.
Ordinarily, in the state before the photographing, this result is periodically (for each frame) transferred to a video memory 111, whereby finder display or the like is performed through a monitor display circuit 112.
On the other hand, when the photographer himself performs the photographing operation by operating the camera operation switch 101, the pixel data of one frame is read from the frame memory 116, objected to image processing by the camera DSP 110, and first stored in a work memory 113, in response to the control signal from the system control CPU 100.
Subsequently, the data in the work memory 113 is compressed based on a predetermined compression format by a compression and expansion circuit 114, and the compressed result is stored in an external nonvolatile memory 117 (ordinarily a nonvolatile memory such as a flash memory is used).
Conversely, when the photographer wishes to observe the picked-up image data after photographing, the data which was compressed and stored in the external nonvolatile memory 117 is expanded into ordinary data of each photographing pixel through the compression and expansion circuit 114, and the expanded result is transferred to the video memory 111, whereby the image data can be observed on the monitor display circuit 112.
As above, the ordinary digital camera is structured to convert the output from the image pickup element 104 into the actual image data through processing circuits substantially in real time, and output the converted result to the memory or the monitor circuit.
On the other hand, in order to improve the capability of running photographing and the like in such a digital camera system as described above (e.g., in order to obtain a capability close to 10 frames/second), systematic improvement including improvement of the image pickup element is necessary. For example, it is necessary to increase a reading speed from the image pickup element, increase a writing speed of the image pickup element data to the frame memory, or the like.
FIG. 2 simply shows, as one of improvement methods, a structure of a two-output type device composed of the image pickup element such as a CCD. In this device, the horizontal CCD is divided into two.
In the CCD of FIG. 2, charges of the respective pixels generated by a photodiode unit 90 are together transferred to the vertical CCD at predetermined timing, and the charge of the vertical CCD for each line is transferred to horizontal CCD""s 92 and 93 at next timing.
Here, the horizontal CCD""s 92 transfer the charges to a left-side amplifier 94 at each transfer clock pulse, and the horizontal CCD""s 93 transfer the charges to a right-side amplifier 95 at each transfer clock pulse, whereby the photographed image data of this CCD is divided into right and left in two on the boundary of the center of the picture, and the divided image data is read.
Although the amplifiers 94 and 95 are ordinarily manufactured within the CCD device, these amplifiers are laid out at positions considerably separated from one another, whereby relative accuracy of the amplifier 94 does not necessarily match with that of the amplifier 95 completely. For this reason, the outputs of the left and right pictures from the amplification are separately sent to CDS/AGC circuits 96 and 98 and adjusted by external adjustment means 97 and 99, respectively, to match the left and right outputs to each other.
In an image pickup element capable of realizing such high-speed reading as described above, a method to simultaneously read the signals from the two or more outputs is applied. This technique is indispensable to bringing the digital camera in the future closer to a silver salt camera (a product having a specification of about 8 frames/second has already been achieved in a silver salt camera of the single lens reflex type).
Having plural outputs is advantageous with respect to the speed. However, from the viewpoint of matching the output levels, having plural outputs is obviously disadvantageous as compared with having only one output.
In the mere manual adjustment method, in which, e.g., the output levels are analog-adjusted in the conventional CDS/AGC circuit, or the output levels after A/D conversion are digital-adjusted by matching both channels, the value of, e.g., VR resistance itself changes due to an environmental change even if the levels have been considerably matched at the manufacturing stage, whereby the possibility to which temperature characteristics of the two CDS/AGC circuits completely match with each other is extremely low.
When such a reading method of the image pickup element as described above is performed, if the relative accuracy of both the left and right outputs exceeds xc2x11%, unbalance of the boundary on the picture is clearly recognized by a viewer.
An object of the present invention is to correct, in an image pickup apparatus which picks up an image of a object by dividing the image into plural image pickup areas, unbalance between image signals output from plural output portions provided for respective image pickup areas.
In order to achieve the above object, according to one aspect of the present invention, there is provided an image pickup apparatus comprising:
image pickup areas for picking up an image of a object by dividing the image into plural areas;
plural output portions, each provided for a respective one of the image pickup areas, for outputting image signals respectively from the image pickup areas;
calculation means for calculating correlation among the plural image signals respectively output from the plural output portions; and
correction means for performing correction processing on the plural image signals output from the plural output portions, on the basis of the signal output from the calculation means.
Other objects and features of the present invention will become apparent from the following detailed description and the attached drawings.